Avatron are about to release a new application for the Apple iPad that will let you use it as a secondary display with your MacBook or Apple Mac, the Air Display iPad App.
The Air Display iPad Application will connect to your Mac via WiFi and extend your display onto the iPad, you will also be able to use the iPad as a mouse and use it to launch applications on your Mac.
The 
Air Display iPad application will be available shortly from iTunes, it certainly seems like a useful application for you iPad and Apple Mac. The application costs $9.99 which is quite a bit cheaper than a secondary display.
